Optically controlled silicon carbide and related wide-bandgap transistors and thyristors

ABSTRACT

An optically active material is used to create power devices and circuits having significant performance advantages over conventional methods for affecting optical control of power electronics devices and circuits. A silicon-carbide optically active material is formed by compensating shallow donors with the boron related D-center. The resulting material can be n-type or p-type but it is distinguished from other materials by the ability to induce persistent photoconductivity in it when illuminated by electromagnetic radiation with a photon energy in excess of the threshold energy required to photoexcite electrons from the D-center to allowed states close to the conduction band edge, which varies from polytype to polytype.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.11/764,606, filed on Jun. 18, 2007, pending, which claims the benefit ofProvisional U.S. Application Ser. No. 60/805,139, filed on Jun. 19,2006, expired. Each of the above applications is incorporated byreference herein in its entirety.

BACKGROUND

1. Field

This invention relates to advanced microelectronic (semiconductor)devices and methods for fabricating the same, and in particular, tomicroelectronic devices containing a region of optically active materialthat permits the device to be closed with a pulse of light of onewavelength; and then opened with a pulse of light of a secondwavelength.

2. Background of the Technology

The circuit shown in FIG. 1 is widely used in diverse applications wherethe conversion of one voltage or current (usually DC) to a three phaseAC voltage or current (or vice versa) is required [1]. Examples includemotor drives for electric vehicles, industrial motors used in factories,and utility power conditioning systems such a staticvolt-ampere-reactive (SVAR) compensators and rectifiers and invertorsused for high-voltage DC electric power transmission, two-switch andfour-switch versions of this circuit (the “half bridge” and the “fullbridge,” respectively) are common in power supply applications usedthroughout the defense and civilian electronics industry.

The circuit has six semiconductor switches that can be constructed inmany forms, including the bipolar junction transistor (BJT), themetal-oxide-semiconductor field effect transistor (MOSFET), theinsulated gate bipolar transistor (IGBT), the static inductiontransistor (SIT), thyristors of the silicon controlled rectifier (SCR)type, the gate-turn-off (GTO) type, or the static induction type [2].Many other variations of the above can be found in the prior art.

The basic circuit building block found in FIG. 1 is the two-switchhalf-bridge phase leg (see FIG. 2). FIG. 2 also shows two disadvantagesof this prior art. The first is commonly known as the “high-side gatedriver” problem in which the upper switch S1 is electrically controlledby gate driver circuitry whose common connection is the load, and thus afloating gate drive is required. This introduces greater complexity andcost into the final system. The second problem is the possibleintroduction of incorrect gate signals which could cause improperoperation of the half-bridge, possibly causing a failure to occur ineither the circuit or the load. The source of these incorrect gatesignals is commonly called “electromagnetic interference” or EMI. EMIcan come from many sources and can effect all applications. But inmilitary related systems, there is the additional threat ofintentionally introduced EMI from enemy action. EMI can effect theoperation of any and all switches in the circuit, including the low-sideswitch S2 in FIG. 2.

Optically controlled circuits represent one remedy to both the high-sidegate driver problem and the EMI problem. FIG. 3 illustrates anotherembodiment of the prior art that partially remedies the problem. Theintroduction of an additional circuit in the gate driver is called anoptical receiver that allows a fiber optic connection between a centralprocessor and any of the switches in the circuit of FIG. 3. The fiberoptic link is generally much less vulnerable to EMI, if not immune.Unfortunately, the problem of providing isolated electrical power to thereceiver and the gate driver remains. And the gate driver circuitry isstill potentially vulnerable to EMI. The former problem is especiallytroublesome whenever a long string of devices are connected in series tomultiply the total blocking voltage of the stack, as is often the casein electric utility equipment.

A typical response is to eliminate, if possible, the gate drivercircuitry all together. The use of optically active switches is onesolution, FIG. 4 reveals additional prior art in which optically activedevices, usually optically triggered thyristors, are used because theydo not need a gate driver to be switched on or “closed.” Generally,optical radiation of a characteristic wavelength generated by a laser(but other sources of optical energy can be used) is conducted bysuitable means (usually fiber optic cable) to the switch. Electron-holepairs are generated in the portion of the switch that is illuminatedsuch that the device switches into conduction [3]. The principallimitation is that the switch cannot usually be switched off with light,which accounts for the popularity of thyristors because they cangenerally be switched off by the external circuit through a processknown as commutation. This limits the optically triggered thyristor, byfar the most commonly used optically active switch used in powerelectronics, to applications where circuit commutation is possible;however, in many applications commutation is not an option whichseverely limits the application of the prior art in optically activeswitches.

Optically active BJTs, also known as phototransistors, are commonly usedin the microelectronics industry in devices such as “optical isolators”(or “opto-isolators” for short) and light detectors of various kinds andapplications. Phototransistors are rarely used in circuits like FIG. 1,but in principal they could be. Phototransistors of the prior art are avariation on the optically triggered thyristor in that electron-holepairs are generated by a light source with a photon energy that exceedsthe bandgap energy of the semiconductor used in the transistor. The baseof the BJT is usually chosen to be the optically active medium. Anadvantage of the optically active BJT is that conduction through thetransistor will continue only for as long as the light shines on thebase of the BJT. When the light is removed, the BJT will stop conductingcurrent and in due course the switch will turn off or “open.” Theproblem is that the delay prior to switch off is generally determined bythe physics of minority carrier storage in the base of the BJT which isgenerally slow for BJTs that have good optical gain [4]. Thephototransistor can be made faster by introducing impurities that resultin a short minority carrier lifetime (MCL) but this negatively impactsthe optical gain. In most applications, the optical energy required toinitiate and sustain conduction is an important figure of merit, whereless is much better.

Similar problems arise in the development of semiconductor switchesintended to control large amounts of transient power, known as pulsedpower generators. These systems are generally found in defense andmedical applications. Very fast switching is demanded by suchapplications [5], which has made semiconductor device development by thepulsed power technical community rather distinct from that developed forapplications in the conventional power electronics community. In thepulsed power community switches that close when illuminated by laserlight and then open when the laser light is removed with a time constantcharacteristic of the material are said to operate in the “linear mode”[6]. Linear-mode switches can be characterized as “light-sustained” bulkphotoconductive closing and opening switches. Such switches are similarin this respect to the phototransistor except that they are simpler inconstruction, often consisting of little more than a block ofsemiconductor, such as silicon or gallium arsenide, with a metal contacton either end to form Ohmic contacts for connecting the switch to theexternal circuit; and their size is typically much larger which reflectstheir completely different application [7]. However, the disadvantageoustrade-off in laser energy for switching speed remains the same [8].

An alternative to the “light sustained” photoconductive switch is taughtby Schoenbach et al. in U.S. Pat. No. 4,825,061 [9], which reveals abulk photoconductive device in which a laser pulse of one wavelengthstimulates persistent photoconductivity which continues for up to manymicroseconds after the laser pulse of nanosecond duration has terminated[10]; and which can be terminated on demand by application of a second“quenching” laser pulse of longer wavelength [11]. Schoenbach et al. in'061 takes advantage of the optical quenching effect which was known by1960 to be particularly strong in Gallium Arsenide doped with copper[12]. The physics of infrared optical quenching in photosensitivesemiconductors like copper-doped GaAs and CdS, the fundamental basis of'061, were adequately understood by 1965 [13]. The teaching ofSchoenbach et al. in '061 is limited to the use of these effects in abulk photoconductive switch whose embodiment is described generally in[7] and [9] and is illustrated in FIG. 5. A substantial literature, e.g.[14] and [15], reveals that the teaching can be practically realized bya photoconductive switch intended for circuits like that shown in FIG. 6and that are generally utilized in pulsed power applications, forexample, as taught by Stoudt et al. in U.S. Pat. No. 5,864,166 [16]. Alldemonstrations of practical working devices have been limited to thebulk photoconductive switch taught by Schoenbach et al. in '061 andfabricated with the same core process of compensating silicon-doped GaAswith copper (GaAs:Si:Cu) by thermal diffusion to make a bulksemi-insulating material [17]. Indeed, no other practical teaching iscontained in '061.

The advantage of the GaAs:Si:Cu photoconductive switch, as compared tothe pulsed power switching prior art, is that it has highphotoconductive gain in a material with short minority carrier lifetime,thus offering a much lower consumption of laser power to applicationsrequiring current pulses with fast rise and fall times and a long and/orcontinuously variable duty ratio. However, as reported in [14] onlyrelatively low average electric fields of the order of 3 kV/cm can becontrolled in GaAs:Si:Cu bulk photoconductive switches because of afundamental instability that leads to current filamentation [18], so toblock large voltages and to conduct large currents, an extremely largeactive area is required with respect to the conventional semiconductordevices used in the power electronics industry. Therefore, prohibitivelylarge laser energy is required to apply the switch to power electronicsapplications. An additional disadvantage is that GaAs is generally apoor choice for power electronics due, among other reasons, to its lowthermal conductivity. Schoenbach et al. does not teach an embodimentthat can be practically applied to a better choice of semiconductor forpower electronics, such as silicon carbide.

SUMMARY

In one aspect, the boron-related D-center is used to compensate shallowdonors in silicon carbide to produce an optically active materialcapable of exhibiting persistent photoconductivity induced by opticalradiation in the yellow or green portion of the electromagneticspectrum, and optical quenching of the same persistent photoconductivitywith optical radiation of longer wavelength in the near infrared to redportion of the electromagnetic spectrum. All of the important polytypesof SiC are rendered with essentially the same properties by compensatingshallow donors with the boron-related D-center, including but notlimited to 3C, 4H, and 6H.

In various embodiments, the optically active material formed by D-centercompensated SiC is incorporated into the appropriate active regions of avariety of microelectronic devices used in power electronicsapplications by selective means. These means include implantation ofboron into silicon carbide substrate material and/or epitaxial materialfollowed by the diffusion of boron resulting in the creation ofD-centers in one-, two-, and three-dimensional device structures viahigh-temperature thermal treatment or via continued epitaxial growth.

In one embodiment, the optically active material formed by D-centercompensated SiC is incorporated in the base of an optically controllablebipolar junction transistor (BJT).

In another embodiment, the optically active material formed by D-centercompensated SiC is incorporated into the channel of opticallycontrollable vertical and lateral channel junction field effecttransistors (JFETs).

In yet another embodiment, the optically active material formed byD-center compensated SiC is incorporated into the channel of anoptically controllable metal-oxide-semiconductor field effect transistor(MOSFET).

In yet another embodiment, the optically active material formed byD-center compensated SiC is incorporated into the p-base of an opticallycontrollable thyristor.

In still another embodiment, the optically active material formed by theD-center compensated SiC is incorporated into the channels formed in thep-base of an optically controllable static induction thyristor (alsoknown as a field controlled thyristor).

The microelectronic devices discussed above, as well as otherconfigurations apparent to others skilled in the art upon examination ofthese teachings, can be incorporated into power electronics systems withthe advantages of permitting pure optical control of both the closingand opening transitions of the switches with virtually infinitecombinations of duty ratios of the closing and opening periods rangingfrom milliseconds for a single closing optical impulse to much longer ifa regular sequence of closing optical impulses illuminate the deviceactive area so as to replenish the persistent photoconductivity.

Additional advantages and features will be set forth in part in thedetailed description that follows and in part will become more apparentto those skilled in the art upon examination of the following or uponlearning of the practice of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a general illustration of the prior art in three-phase motordrives.

FIG. 2 is an illustration of the prior art in conventional half-bridgecircuits employing electrically controlled gate drivers and electricallycontrolled switches.

FIG. 3 is an illustration of the prior art in conventional half-bridgecircuits employing optically controlled gate drivers and electricallycontrolled switches.

FIG. 4 is an illustration of the prior art in optically controlledhalf-bridge circuits employing optically stimulated or sustainedswitches.

FIG. 5 is an illustration of the prior art in GaAs:Cu:Si photoconductiveswitches (after Ref. [15]).

FIG. 6 is an illustration of the application and results of using theprior art in GaAs:Cu:Si photoconductive switches in a pulsed powercircuit (after Ref. [15]).

FIG. 7 is an illustration of the application of one or more embodimentsof the present invention in a half-bridge circuit.

FIG. 8 is an illustration of the bandgap of the optically activematerial consisting of the 6H polytype of the silicon carbidesemiconductor doped with shallow nitrogen donors and compensated withboron acceptors and boron-related D-centers (6H—SiC:B:N) and containedin one or more embodiments of the present invention.

FIG. 9 is an illustration of the cycle of photoconductivity possible inan optically active material made from SiC:B:N compensated withD-centers when pulses of electromagnetic radiation of two differentwavelengths consecutively illuminate the optically active region of oneor more embodiments of the present invention.

FIG. 10 is a cross-sectional view of a vertical planar bipolar junctiontransistor (BJT) containing an optically active region of SiC:B:Nilluminated from the ends in accordance with an embodiment of thepresent invention.

FIG. 11 is an illustration of the cycle of charge control possible in anoptically active material made from SiC:B:N compensated with D-centerswhen laser pulses of two different wavelengths consecutively illuminatethe optically active region of the bipolar junction transistorillustrated in FIG. 10.

FIG. 12 is a cross-sectional view of a vertical trench bipolar junctiontransistor (BJT) containing an optically active region of SiC:B:Nilluminated from the ends through lossy waveguides fabricated in thetrenches in accordance with an embodiment of the present invention.

FIG. 13 is a cross-sectional view of a vertical trench bipolar junctiontransistor (BJT) containing an optically active region of SiC:B:Nilluminated from the top into lossy waveguides fabricated in thetrenches in accordance with an embodiment of the present invention.

FIG. 14 is a cross-sectional view of a vertical channel junction fieldeffect transistor (JFET) containing an optically active region ofSiC:B:N illuminated directly from the top in accordance with anembodiment of the present invention.

FIG. 15 is a cross-sectional view of a lateral channelmetal-oxide-semiconductor field effect transistor (MOSFET) containing anoptically active region of SiC:B:N illuminated directly from the top inaccordance with an embodiment of the present invention.

FIG. 16 is an embodiment of the current invention in which the opticallyactive device shown in FIG. 14 is used to drive the gate of aconventional electrically gated power device.

DETAILED DESCRIPTION

The present invention provides for a silicon carbide optically activematerial used to create an entire class of power devices and integratedcircuits with significant advantages in circuits requiring or benefitingfrom control by pure optical gating. An example of one such applicationis the circuit 1 shown in FIG. 7. The circuit is a half-bridge powercircuit containing two transistor switches 2, one in the “high side”position and one in the “low side” position. A controller 3 drives asource of electromagnetic radiation 4 which can produce beams ofelectromagnetic radiation of one wavelength 5 and of a second wavelength6 such that the first wavelength is less than the second wavelength.Both the high-side and the low-side switches can be illuminated witheither of the beams in arbitrary sequences as determined by thecontroller. Such half-bridge circuits would have application bythemselves in various electrical power supplies, or as output stages inintegrated circuits. Or two of them could be connected together to formfull-bridge circuits (not shown). Or three of them could be connectedtogether to form three-phase circuits as illustrated in FIG. 1. Or otherinterconnections of these circuits could become apparent to thoseskilled in the art upon examination of the following or upon learning bypractice of the invention.

The optically active material in some embodiments is based on siliconcarbide semiconductor from any of the known polytypes, the most commonof which are 4H—SiC and 6H—SiC. However, 3C—SiC, 15R—SiC, as well asothers will work equally well. Silicon carbide is a class ofsemiconductors with wide bandgaps. For example, 4H—SiC has a bandgap ofabout 3.2 eV at T=300 K, while 6H—SiC has a bandgap of about 2.9 eV atT=300 K. SiC is also an indirect bandgap semiconductor. The wide bandgappermits high-voltage and high-temperature operation atcharacteristically low specific on-resistance as compared to narrowbandgap semiconductors such as silicon or gallium arsenide. Also, SiC ofany polytype has an advantageously large thermal conductivity (at leastsix times that of GaAs). Thus, the use of SiC devices is growing inapplications requiring power semiconductor devices, such as theimportant power electronics industry.

Specifically, the optically active material is SiC doped with shallowdonors, such as nitrogen or phosphorus, and compensated with boronacceptors and boron-related D-centers. A diagram of the bandgap 10 of anexample of this material in 6H SiC is shown in FIG. 8. 6H—SiC has aconduction band 11 separated from the valence band 12 by an energy of2.9 eV. The 6H—SiC material is doped during growth by nitrogen, whichforms shallow donor levels 13 averaging 0.1 eV below the conductionband. Either during growth, or most likely afterward, boron isintroduced into the 6H-1-SiC. Boron forms two distinct types of impuritycenters in SiC. The first is the boron acceptor 14 at about 0.3 eV abovethe valence band which is formed when a boron impurity atom substitutesonto a silicon vacancy site. The second is the D-center which forms whena boron atom substitutes onto a silicon vacancy site that is part of alarger complex of native point defects, such as the so-called carbonvacancy V_(C). The D-center is definitely deeper in the bandgap than isthe boron acceptor and any particular SiC polytype can containconcentrations of both boron acceptors and D-centers. The D-center hasbeen well studied both with optical spectroscopic means and by thermalspectroscopic means. The result is reported as an optical activationenergy of 0.73 eV above the valence band and a thermal activation energyranging from 0.58 to 0.63 eV above the valence band [19]. FIG. 8illustrates one resolution of this discrepancy by use of the two-levelmodel found in Ref. [19] in which a D-center ground state 15 is locatedat 0.73 eV above the valence band while a second excited state of theD-center 16 is located at about 0.58 eV above the valence band. Theobserved difference between the optical and thermal activation energiesassociated with electron and hole capture at the D-center are predictedby this model. The material can be optically active when either shallowdonor states outnumber the sum of the boron and D-center acceptor states(leaving the material n-type when in thermal equilibrium), or when theopposite is true (leaving the material p-type when in thermalequilibrium).

FIG. 9 illustrates the cycle of photoconductivity that can be excited byoptically active SiC:B:N material. FIG. 9( a) shows the equilibriump-type material in its highly resistive state in which little or noconductivity is observed. Virtually all of the nitrogen donor states arepositively charged and thus contain a trapped hole 20. Virtually all ofthe boron acceptor states and many of the D-center states are negativelycharged and thus contain a trapped electron 21. The remaining neutralD-center states contain a trapped hole 20.

FIG. 9( b) shows the material when illuminated by electromagnetic energywith wavelength less than 580 nm. Photons with this energy (>2.14 eV)will excite process 30 in which electrons are photoionized from theD-center ground state 15, leaving a free electron 22 and a trapped holein the neutral D-center. As long as the wavelength exceeds 500 nm, therewill be not be electron photoionization of the boron acceptor. Photonswith this energy will also excite process 31 in which trapped holes arephotoionized from the D-center ground state leaving a free hole 23 and atrapped electron in the negatively charged D-center. Because both theB-acceptor and the D-center have relatively large hole capture crosssections of between 0.1 and I×10⁻¹⁴ cm² [20] free holes are likely to betrapped by both of these centers (process 32) where they can be excitedback into the valence band by another photon absorption process 31. TheD-center has a much smaller electron capture cross section as revealedby D-center photoluminescence glow times of one minute atlow-temperature [19]. Consequently, it is unlikely that a photoionizedelectron will be recaptured. The result is persistent photoconductivity(PPC), as illustrated in FIG. 9( c). When the source of electromagneticradiation is removed, remaining holes in the optically active materialare trapped by the D-center and the B acceptors through process 32. Thenon-equilibrium free electrons remain in the conduction band untileither they are captured by the D-center (process not shown) through acharacteristic emission of a broad band of photons centered at about2.13 eV in 6H [20] (or 2.34 eV in 4H [21]) or they recombine with a holethat is thermally emitted by the D-center (process 33 followed byprocess 34). The combination of processes 33 and 34 is known to occurand is called thermal quenching [19]. However, the time constant of theprocess is governed by the relatively slow thermal emission of holesfrom the D-center, which has been observed in numerous reports to occuron a time scale of about 10 ms at 300 K (e.g., [20] and [22]).Persistent photoconductivity of this duration is about 1000 times longerthan that observed in GaAs:Cu:Si [10], and thus represents a significantimprovement over the prior art.

Persistent photoconductivity can be optically quenched as shown in FIG.9( d), when electromagnetic radiation with a wavelength less than 1.77μm but greater than 0.58 μm illuminates the optically active material.Photons in this energy range (0.7<hv<2.13 eV) excite process 35 but notprocess 30. This means that holes that were being thermally emitted at avery slow rate by process 33 will now be emitted at a rate determined bythe much greater rate of process 35. Assuming the recombination process34 is of a comparable rate as hole capture by process 32, then quenchingof the PPC will occur. If the recombination process is significantlyslower than hole capture, then optical quenching will not be observed.Therefore, the present invention includes an optically active materialwith an electron-hole recombination rate comparable to or faster thanthat of the hole capture process 32. In that case the material will bereturned to the state shown in FIG. 9( e), which is substantiallysimilar to the initial state shown in FIG. 9( a).

FIG. 10 is an illustration of the cross section of one embodiment ofthis invention comprising a bipolar junction transistor (BJT) with anoptically active region. The device is fabricated on an n-typeconducting substrate 40. Upon the substrate is grown epitaxially ann-type collector 41 of sufficient thickness and doping to block therequired voltage while the device is in the off state. Upon thiscollector is fabricated an optically active region of semiconductor 42that acts as the base of the BJT. Methods for fabricating such anoptically active material can be fabricated by compensating an epitaxiallayer containing a certain concentration of shallow donors byintroducing boron in a way that forms D-centers as disclosed in U.S.Patent Application 2002/0149021 A1 [23]. The layer 42 is sufficientlycompensated with boron acceptors and D-centers to become highlyresistive and p-type in conductivity. An n-type emitter layer 43 isadded on top of layer 42 either by epitaxial means or by ionimplantation. This embodiment is especially compatible with the deepmesa edge termination technique which is shown in FIG. 10 with asidewall passivating dielectric material 46 applied Ohmic contact to theemitter layer is formed by a suitable sequence of metal layers and heattreatments leaving a metal stack 44 as the emitter contact. A similarprocess is applied to the bottom of the substrate 40 leaving a metalstack 45 as the collector contact. Light of one wavelength or the otheris introduced at the edges of the device. As the light propagatesthrough the device from one or more sides, processes 30 or 35 occur inthe optically active region 42 which can change the conductivity of thismaterial in a way that changes the switch state of the device. In thecase of electromagnetic radiation with a wavelength that producesprocess 30 the device state is changed from non-conducting (“blocking”)to conducting (“on”). In the case of electromagnetic radiation with awavelength that produces process 35 only, the device state is changedfrom conducting (“on”) to non-conducting (“blocking”).

The ability to activate the entire volume of optically active materialin a power device of significant dimension is a significant advantage ofthis invention. The wavelengths of the electromagnetic radiationdisclosed in the physical description of the photoconductivity cycle inFIG. 9 are known as sub-bandgap wavelengths because the photon energy ofthe radiation is less than the bandgap of the semiconductor. Therefore,the radiation is much more weakly absorbed than in the case ofabove-bandgap wavelengths. The formula for the characteristicpenetration of electromagnetic radiation into a material is given by Eq.(1):I(x)=I ₀ exp(−αx)  (1),

where I(x) is the intensity of the radiation in units of W/cm² at apoint x inside of the optically active material 42, I₀ is the initialintensity of the radiation at the surface of the BJT where the radiationis introduced into the device (x=0), and a is the absorption coefficientof the radiation at the specified wavelength in units of cm⁻¹. Forsub-bandgap radiation, the absorption coefficient is determined byphotoionization of deep levels, like the D-center. An approximation tothe absorption coefficient is given by Eq. (2):α=σN _(D)  (2),

where σ is the cross section for photoionization in units of cm² andN_(D) is the number density of D-centers in units of cm⁻³. It has beenreported that α=4.17×10⁻¹⁷ cm² for the boron-related absorption bandwith a threshold photon energy of 0.7 eV [24], which is the D-center.The characteristic absorption depth of the sub-bandgap electromagneticradiation isd=1/α.  (3).

The embodiment of FIG. 10, in which the electromagnetic radiation ispropagated through the length of the device, requires that d becomparable to the lateral dimensions of the device so that the photonsare absorbed efficiently and uniformly throughout the optically activematerial. Such a large distance means the SiC is nearly transparent tosub-bandgap wavelengths. Typically, it is preferable for d>1 mm, whichmeans N_(D)<10¹⁷ cm⁻³.

Understanding the teachings of this disclosure requires more than simplyconsidering the photoconductive cycle shown in FIG. 9 and the teachingspresented in '061 by Schoenbach. The incorporation of an opticallyactive material into a practical semiconductor device of the type shownin FIG. 10 is not taught in the prior art and is a non-obviousimprovement.

FIG. 11( a) illustrates one-half of a BJT of the embodiment shown inFIG. 10. The BJT is shown with a voltage V_(C) applied to the collectorcontact and a voltage V_(E) applied to the emitter contact, whereV_(C)>V_(E). The optically active material is in the quasi-equilibriumstate illustrated by FIG. 9( a) and will thus be partially depleted offree holes, leaving a region of negative space charge 60. Likewise, then-type material in contact with the optically active material at themetallurgical junction formed between the two materials is partiallydepleted of free electrons, leaving a region of positive space charge61. The BJT in this state will allow only a small leakage current toflow upon application of a differential voltage V_(CE)=V_(C)−V_(E) up tothe dielectric breakdown strength of the device.

FIG. 11( b) illustrates the change in the device caused by illuminationby electromagnetic radiation with wavelength short enough to excite thephotoionization processes 30 and 31. The net result is the creation ofsignificant densities of free electrons and free holes in the opticallyactive material. These free carriers separate in the electric fieldgenerating an electrical current 62 by drift and diffusion. Electronsand holes that drift to the collector and emitter contacts,respectively, are replaced by new photoionization events in theoptically active material. This gives rise to a much larger currentflowing from collector to emitter than existed while the devices was inthe blocking state shown in FIG. 11( a). Since the process of photoabsorption is among the fastest known to modern physics, the change ofthe BJT from the blocking state to the conducting state can occur overthe time scale of the pulse of electromagnetic radiation, which caneasily be nothing more than nanoseconds in duration.

Eventually, the pulse of electromagnetic radiation subsides and theremaining free holes are trapped (process 32 in FIG. 9( b)) into boronacceptors and D-centers. The result is that the optically activematerial has been optically converted from an equilibrium p-typematerial to the non-equilibrium n-type material illustrated in FIG.11(c). The optically active material now acts like a p-base of a BJT inwhich the holes are immobile (as indeed they are because they have beentrapped in the hole traps) so that they cannot be injected into theemitter and they cannot recombine with the electrons injected from theemitter 63. The loss of injected electrons is determined by the rate ofthe hole-emission process 33 in FIG. 9( c). Since that rate is very lowfor the D-center, the device remains in the non-equilibrium state ofFIG. 11( c) for some 10 ms at T=300 K, and while in that state it actsas a very high-gain BJT for which currently there is no conventionalequivalent in SiC. For conducting current longer than 10 ms, a secondpulse of electromagnetic radiation with wavelength sufficiently short toinduce processes 30 and 31 can be applied as often as necessary tosatisfy the desired conduction period.

The persistence of the non-equilibrium conducting state shown in FIG.11( c) can be terminated on command at anytime by illuminating thedevice with electromagnetic radiation with a wavelength sufficient toinduce process 35 in the optically active material as shown in FIG. 11(d). This process produces only free holes, which largely participate inone of three processes while in the valence band. One process is to berecaptured as in process 32 of FIG. 9( b). A second is process 64 inwhich the photoionized hole drifts and/or diffuses to the emitter layer,which leaves behind a quanta of negative space charge. A third isprocess 65 in which a photoionized hole recombines with a free electron,either spontaneously or with the assistance of a recombination center inthe bandgap of the SiC. The latter is typically much faster in anindirect semiconductor like SiC and thus can be expected to dominate. Inthis way, minority carrier charge in the form of electrons is rapidlydepleted from the optically active material leaving a negative spacecharge region 60. Likewise, the drift and/or diffusion of a freeelectron to the collector contact in the n-type material (process 66)that is not replaced from the optically active material leaves behind aquanta of positive charge; and in this way the n-type material adjacentto the junction is simultaneously and rapidly depleted of free electronsleaving behind a positive space charge region 61. Consequently, thedevice is returned to the blocking state shown in FIG. 11( e).

The embodiment shown in FIG. 10 has many advantages, including thesimplicity of manufacturing the semiconductor device and the large ratioof the active area to the physical area of the device. One disadvantageis the problem of propagating light efficiently and uniformly from theedges of the device throughout the volume of optically active material.

The embodiment shown in FIG. 12 offers an improvement in this respect.This device is also a BJT, but with a trenched cross section instead ofa planar cross section. The device is formed on a conducting n-typesubstrate 40 by epitaxially growing an n-type drift layer 41. An n-typechannel layer 47 is grown over the drift layer 41 followed by an emitterlayer 43 that is grown or implanted onto or into, respectively, thechannel layer. Trenches are formed such that their depth exceeds thethickness of the emitter layer 43. Boron is implanted into the trencheswhile prevented from being implanted into the emitter layer by asuitable masking material, and thus representing a self-aligned process.Such an embodiment also incorporates teachings found in thespecification and claims of U.S. Pat. No. 6,767,783 [25]. The boronimplanted region 48 represents a solid source of boron selectivelyplaced into the trench structure. An additional thermal process step ofsufficient temperature will cause boron to diffuse into the channellayer and introduce boron acceptors and D-centers, thus selectivelyproducing a region of p-type optically active material 42. The trenchesare filled with a combination of dielectric materials 49 that serve twopurposes. First, the dielectric materials provide surface passivationand electrical insulation of the emitter base junction formed betweenthe emitter layer and the p-type optically active material. Thedielectric stack also forms a lossy waveguide of electromagneticradiation at optical frequencies, including the infrared band and thevisible band. To finish the device, an edge termination structure isformed, such as the deep mesa with dielectric passivation structure 46illustrated. Ohmic metal contacts are added to form the emitter contact44 and the collector contact 45.

As shown in FIG. 12, when beams of electromagnetic radiation 36 arelaunched down the trenches from one or both ends of the device, some ofthe radiation scatters into the optically active material on either sideof the trench, producing processes 30 and 31 or process 35 depending onthe wavelength of the radiation. The BJT is switched from thenon-conducting state to the conducting state and back to thenon-conducting state in a sequence essentially identical to that shownin FIG. 11. The width of the channel fingers can be optimally chosen bythose skilled in the art to fall in a range from one micrometer to manymicrometers, depending on the channel doping and the absorption lengthof the electromagnetic radiation.

Both of the embodiments shown in FIG. 10 and FIG. 12 still require theelectromagnetic radiation to be introduced from the edges of the device,which increases the complexity of the interface between the device andthe source of the electromagnetic radiation. FIG. 13 is yet anotherembodiment of the invention that is a BJT substantially similar to theembodiment shown in FIG. 12 except that the emitter contact 44 hasopenings fanned such that electromagnetic radiation can be launched intothe waveguide 49 from the top of the device. The radiation is againscattered into the optically active material 42 as with the embodimentshown in FIG. 12. This, and all embodiments that are excited withelectromagnetic radiation from the top, are highly compatible with manydifferent edge termination techniques in addition to the deep mesatechnique shown in FIG. 13, including (but not limited to) raised andburied guard rings and the junction termination extension (JTE) in itsmany forms.

Yet another embodiment modifies the BJT device structure to add ajunction field effect transistor (JFET) structure as shown in FIG. 14.The device is formed of a conducting substrate 40 and an n-type driftlayer 41 as in previous devices. A layer of strongly p-type material 50is added using a dopant other than boron, such as aluminum. The layercan be formed either by epitaxial means or by implantation. A boronimplant 48 is added to the surface of the p-type layer 50. Trenches areformed through the p-type layer and a channel is regrown using epitaxialmeans to fill the trenches and cover the p-type layer and the boronimplant using methods that leave a substantially planar surface. On topof the channel a heavily doped n-type layer 43 is also grown. Such anembodiment also incorporates teachings found in the specification andclaims of [26]. During this regrowth, boron diffuses into the growingchannel forming boron acceptors and D-centers and converting the channelinto a p-type optically active material 42. Ohmic metal is added to thetop of the source layer 43 and the bottom of the substrate 40 to formsource contact 44 and drain contact 45, respectively. The source contactis patterned by photolithographic means to open windows to allowelectromagnetic radiation to pass through the source layer 43 andstimulate in the optically active material 42 processes 30 and 31 orprocess 35, depending on the wavelength of the radiation. This causesthe device to switch from a non-conducting state to a conducting stateand back to a non-conducting state similarly to that shown in FIG. 11.The source contact 44 also makes electrical contact with the p-typelayer 50 to ensure that the potential between the source and this p-typelayer is zero.

It may be desirable in some applications for the depleted portion of thesemiconductor device at the boundary of the drift region and theoptically active material to not be significantly illuminated byelectromagnetic radiation. In FIG. 15 yet another embodiment isillustrated in which the JFET channel region is shorted by a heavilyn-type material 51. The conduction of current through the device is thuscontrolled solely by the conducting state of the optically activematerial 42 at the surface of the device. In this embodiment, a windowis opened in both the source contact 44 and the source layer 43. Thesurface of the channel between the source material 43 and the channellayer 51 is in contact with the dielectric passivation material 46. Thisembodiment is similar to an un-gated metal-oxide semiconductor fieldeffect transistor (MOSFET). The window through the source contact andthe source material allow the optically active material to be stimulatedby electromagnetic radiation from the top and thus the device operatessimilar to the embodiment shown in FIG. 14.

An important consideration for determining whether the present inventionhas practical application is the amount of energy in each pulse ofelectromagnetic radiation required to cause the device to changeconducting state. A kinematic approach is taken to estimate a figure ofmerit, which is the optical energy per ampere of current required.

The energy per unit area is given by Eq. (4):E/A=ηq hvN _(D)  (4),

where E/A is energy per unit area in each pulse of electromagneticradiation in units of J/cm², η is the dimensionless multiplicationfactor required to address various inefficiencies of optical excitation,q is the elementary charge=1.60×10⁻¹⁹ C, hv is the energy per photon inunits of eV, which for 4H—SiC is estimated to be 2.4 eV, and N_(D) isthe effective maximum number density of electrons in units of cm⁻³ thatcan be excited into persistent photoconductivity, which is approximatelygiven by the shallow donor density in the optically active material.

The figure of merit is computed by normalizing Eq. (4) with the currentdensity to be conducted by the device while in persistentphotoconductivity. The rated current of a typical power device isspecified at a forward voltage of 2 V. If the specific on resistance ofthe device is at about the state of the art for 4H—SiC power JFETs, thenρ_((on))=2.5 mΩ-cm² [27]. The expected rated current density for thistechnology isJ=V _((on))ρ_((on))=2 V/2.5 mΩ-cm²=800 A/cm².  (5)

By normalizing Eq. (4) with Eq. (5) the figure of merit is written as:(E/A)/J=ηq hvN _(D)ρ_((on)) /V _((on))  (6)

For N_(D)=1×10¹⁶ cm⁻³ about the correct value for a 600-V device withρ_((on))=2.5 mΩ-cm², and ignoring for the moment, then (E/A)/J=2 nJ/A.The multiplication factor η cannot be less than 2 in a materialoptimized only for turn on, and if the efficiency is balanced for goodturn off performance as well, then the multiplication factor may be aslarge as 10. The frequency of electromagnetic pulses that must beapplied to maintain conduction depends on the thermal hole emissionrate, which at room temperature is quite low at 100 s⁻¹ [20]. But as thejunction temperature increases, so does the rate of hole emission. AtT=−200° C., the hole emission rate will increase about 10,000 fold andthe characteristic hole emission time becomes about 1 μs. Since therepetition frequency of the typical power electronics application isless than 1 MHz, then it can be assumed that in a practical applicationthe time interval between pulses of electromagnetic radiation at workingtemperature is governed by the hole emission rate. A pulse repetitionfrequency (PRF) of 1 MHz multiplied by the figure of merit (including anefficiency multiplier of 10) means that the required average power ofthe source of electromagnetic radiation is about 2 nJ/A×10×10⁶ Hz=20mW/A. So, for example, a 100-A 4H—SiC BJT device at 200° C. requiresabout 2 W of optical power to sustain conduction. This is quitepractical, but it is not insignificant.

Another approach to achieve insertion into an application with the samefunctionality as shown in FIG. 7 is to pair a small-scale version of adevice with the embodiment of this invention and a full-scale powerdevice of conventional construction. Such a full scale conventionaldevice might be made from silicon carbide, or it might be made fromsilicon, or yet another material among the many semiconductors. If sucha conventional device were a normally off device (a so-called“enhancement mode” device), then the circuit shown in FIG. 16 representsanother embodiment of the invention because if the components revealedin FIG. 16 are packaged together, then a device results that isessentially indistinguishable from a monolithic embodiment of theinvention as revealed in FIGS. 10, 12, 13, 14, and 15.

In FIG. 16, the power device of conventional construction 2 can be aBJT, an enhancement-mode MOSFET, a normally off JFET, or a gate turn-off(GTO) thyristor, or any other similar power device. When a pulse ofelectromagnetic radiation 5 illuminates an optically controlled JFET 7of similar construction as shown in FIG. 14, it switches positive gatebias to the power device 2 from an energy storage network 8 consistingof an element that stores electrical charge and an element that permitsthe flow of current in only one direction, such as a diode. The diodeallows the electrical storage device to be charged by the source ofvoltage being switched by the power device 2, but not to be dischargedby the power device. The network 9 that is connected between the gateand common terminal of the power device performs two functions. First,it limits the voltage that can be applied to the gate with respect tothe common terminal to a safe value. In this function it is assisted bythe optically controlled JFET 7 which is capable of self-limiting thecurrent to the network 9 even when the voltage across the charge storageelement in network 8 is as large as the rated blocking voltage of thepower device 2. Second, it maintains a path for the charge stored in thepower device to be discharged from the gate to the common terminal ofthe power device. One example of a network 9 is one formed by a voltagelimiting diode, known as a Zener diode, connected in parallel with aresistor. Alternatively, the resistor can be replaced with a device thatacts like a current source, such as a JFET with gate and sourceterminals shorted together. When a pulse of different wavelength 6illuminates the optically controlled JFET 7 then the JFET turns off andthe gate of the power device 2 is discharged by the network 9 causingthe power device to turn off. As shown in FIG. 16, the controller 3 isable to control the conventional power device with an opticaltransmitter 4 in the same way as shown in FIG. 7; albeit with anadvantageous reduction in the optical energy required.

The gain from the embodiment shown in FIG. 16 can be estimated byconsidering the reduction in the size of the optically controlled devicewith respect to that of the full-scale power device. A figure of meritthat describes the power handling capability of a power switching deviceis the product of the on-resistance of the device R_((on)) and the gatecharge Q_(G) required to switch the device into conduction down toR_((on)). The best SiC power JFET devices have an R_((on)) Q_(G) productequal to about 5×10⁻⁹ ΩC at 200° C. The rated drain current of the powerdevice is I_(D)=V_((on))/R_((on)). The average gate current that must besupplied by the optically controlled device is 1_(G)=PRF×Q_(G).Therefore,R _((on)) Q _(G)=(V _((on)))/PRF)×(I _(G) /I _(D))  (7),

where the optical gain is equal to the reciprocal of the ratio of thearea of the optically controlled device to that of an equivalentoptically controlled device equal in size to the conventional powerdevice, which is equal to the ratio I_(G)/I_(D). Solving (7) forI_(G)/I_(D) and assuming V_((on))=2 V and PRF=1 MHz thenI_(G)/I_(D)=0.0025. The optical gain is the reciprocal of this ratiowhich is 400. If in the previous example the embodiment shown in FIG. 16is used, then to switch the same 100 A at 200° C. the required opticalpower is reduced by a factor of 400 to 5 mW. Appropriate sources ofpulsed electromagnetic radiation capable of supplying average power ofthis magnitude are easily available and relatively inexpensive.

Example embodiments of the present invention have now been described inaccordance with the above advantages. It will be appreciated that theseexamples are merely illustrative of the invention. Many variations andmodifications will be apparent to those skilled in the art.

Furthermore, the purpose of the Abstract is to enable the U.S. Patentand Trademark Office and the public generally, and especially thescientists, engineers and practitioners in the art who are not familiarwith patent or legal terms or phraseology, to determine quickly from acursory inspection the nature and essence of the technical disclosure ofthe application. The Abstract is not intended to be limiting as to thescope of the present invention in any way.

REFERENCE

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What is claimed is:
 1. A method of making a semiconductor devicecomprising: forming a first layer of n-type SiC semiconductor materialon an n-type conducting substrate; forming an optically active,semi-insulating SiC region on the first layer of n-type SiCsemiconductor material; forming a second layer of n-type SiCsemiconductor material on the optically active, semi-insulating SiCregion; selectively etching through the second layer of n-type SiCsemiconductor material, the optically active, semi-insulating SiC regionand the first layer of n-type SiC semiconductor material in a peripheralregion of the device to form a mesa having sidewalls; and depositingdielectric material on the sidewalls of the mesa.
 2. The method of claim1, further comprising: forming an ohmic contact on the surface of thesecond layer of n-type SiC semiconductor material opposite the opticallyactive, semi-insulating SiC region; forming an ohmic contact on thesurface of the substrate opposite the first layer of n-type SiCsemiconductor material.
 3. The method of claim 1, wherein forming theoptically active, semi-insulating SiC region on the first layer ofn-type SiC semiconductor material comprises doping the first layer ofn-type SiC semiconductor material with boron such that boron-relatedD-center defects are formed in the first layer.
 4. A method of making asemiconductor device comprising: forming a first layer of n-type SiCsemiconductor material on an n-type conducting substrate; forming asecond layer of n-type SiC semiconductor material on the first layer ofn-type SiC semiconductor material; forming a third layer of n-type SiCsemiconductor material on the second layer of n-type SiC semiconductormaterial; selectively etching through the third layer of n-type SiCsemiconductor material to expose portions of the underlying second layerof n-type SiC semiconductor material thereby forming trenches;selectively implanting boron in the exposed portions of the underlyingsecond layer of n-type SiC semiconductor material at the bottom of thetrenches to form optically active, semi-insulating SiC regions in thesecond layer of n-type SiC semiconductor material; and depositingdielectric material in the trenches.
 5. The method of claim 4, furthercomprising: depositing one or more metal layers on the third layer ofn-type SiC semiconductor material and on the dielectric material in thetrenches.
 6. The method of claim 5, further comprising forming openingsin the one or more metal layers over the dielectric material.
 7. Themethod of claim 6, further comprising: selectively etching through thethird layer of n-type SiC semiconductor material, the second layer ofn-type SiC semiconductor material and the first layer of n-type SiCsemiconductor material in a peripheral region of the device to form amesa having sidewalls; and depositing dielectric material on thesidewalls of the mesa.
 8. A method of making a semiconductor devicecomprising: forming a drift layer of n-type SiC semiconductor materialon an n-type conducting substrate; forming a layer of p-type SiCsemiconductor material on the drift layer, wherein the p-type layer isdoped with a dopant other than boron; implanting boron in the layer ofp-type SiC semiconductor material; selectively etching through the layerof p-type SiC semiconductor material to form first etched regions;epitaxially growing a channel layer of n-type SiC semiconductor materialin the etched regions and on the surface of the layer of p-type SiCsemiconductor material adjacent the etched regions, wherein borondiffuses into the channel layer during epitaxial growth of the channellayer to form one or more optically active, semi-insulating SiC regionsin the channel layer; and epitaxially growing a source layer of n-typeSiC semiconductor material on the channel layer.
 9. The method of claim8, further comprising: forming a source ohmic contact on the sourcelayer; and forming a drain ohmic contact on the substrate opposite thedrift layer, wherein the source ohmic contact is in electrical contactwith the layer of p-type SiC semiconductor material.
 10. The method ofclaim 9, further comprising: selectively etching through the sourceohmic contact to form second etched regions.
 11. The method of claim 10,further comprising depositing dielectric material on the source layer inthe second etched regions.
 12. The method of claim 11, furthercomprising selectively etching through the source layer to form thirdetched regions.
 13. The method of claim 12, further comprisingdepositing dielectric material on the channel layer in the third etchedregions.
 14. The method of claim 9, further comprising: selectivelyetching through the layer of p-type SiC semiconductor material and thedrift layer in a peripheral region of the device to form a mesa havingsidewalls; and depositing dielectric material on the sidewalls of themesa.